Recent advances in ultrafast laser and supercontinuum generation technology have enabled the development of novel types of compact fiber based ultrafast lasers and frequency combs operating above 2 microns. The first prototypes of these lasers have successfully gone through the industrial feasibility studies and are entering the market.
These sources are characterized by ultrashort pulse duration down to few optical cycles, Watt level output power, tens of nanojoules pulse energy, and up to GHz repetition rate. Important aspect of the new technology is the tunability of the laser output. The emission wavelength of the femtosecond laser is selectable by the customer in the wide spectral range, roughly from 2 to 3 um. In the case of all-fiber design, the emission wavelength could be also made electronically tunable up to 2.5 micrometres, while keeping the pulse duration in femtosecond range. Alternatively, if demanded by application, the laser can be configured as a coherent supercontinuum light source with spectrum reaching wavelengths well beyond 3 um.
The built-in tunability of femtosecond pulses, high quality frequency combs as well as the ability to produce supercontinua directly from the laser allows using these light sources in a range of new and exciting application areas in science and industry. The application areas include, but are not limited to microelectronics, photovoltaics, THz generation, confocal nonlinear microscopy and surgery, as well as environmental, oil and gas sensing.
The talk will overview the proprietary ATLA Lasers AS laser technology of the mid-infrared ultrashort pulse lasers and supercontinuum sources. Focus will be made on the specific aspects of the technology making it particularly attractive for industrial applications demanding either high quality processing or ultrahigh sensitivity measurements.
Proc. SPIE. 10088, Nonlinear Frequency Generation and Conversion: Materials and Devices XVI
KEYWORDS: Mid-IR, Optical parametric oscillators, Femtosecond phenomena, Frequency combs, Nonlinear dynamics, Harmonic generation, Frequency conversion, Sum-frequency generation, Femtosecond frequency combs, Current controlled current source
Half-harmonic generation is the reverse of second harmonic generation that happens in optical parametric oscillators (OPOs) at degeneracy. It is an intrinsically phase-locked down-conversion process, which can be used to efficiently transfer well-developed near-IR frequency combs to the mid-IR. <p> </p>We overview recent experimental progress in cascading multiple stages of half-harmonic generation of femtosecond frequency combs starting from a 1-μm pump. We have achieved stable operation with efficiencies as high as ~64%, pulses as short as three optical cycles at 4 μm, and output powers as high as 2.6 W at 2 μm. Our recent numerical and analytical studies of nonlinear dynamics and different operation regimes of femtosecond OPOs indicate a path toward achieving even higher efficiencies and shorter pulses.
We report supercontinuum generation in the 1.7-2.9 μm range with up to 3.08 W of output power and in the range of 1.93-3.18 μm with up to 3.8 W of output power from all-fiber MOPA pulsed systems with Tm-doped fiber mode-locked seed laser. Supercontinuum generation was demonstrated in nonlinear germanate fibers and fluoride (ZBLAN) fibers. The supercontinuum bandwidth reached 1250 nm at -10 dB level.
We report the solid-state Cr:ZnS laser mode-locked by CNT-based saturable absorber. The absorber was deposited on a protected silver mirror used as a high reflector mirror in a standard 250-MHz cavity with chirped mirror GDD compensation. Laser pulses with duration of 61 fs were obtained at 2.35 μm wavelength. The output power was limited at 950 mW, corresponding to the pulse energy of 3.8 nJ. We have demonstrated the longest-wavelength mid-IR CNT-mode-locked laser with record parameters, advancing the carbon nanotube mode-locking technology well beyond 2 μm into the mid-IR.
A room-temperature Kerr-Lens modelocked (KLM) Cr:ZnS laser generates <70 fs pulses duration (about eight optical
cycles) with 5.6 nJ pulse energy and over 100 nm FWHM spectral width at 105-157 MHz repetition rates. The laser
produces 1 W average output power at 20% optical efficiency, limited by the available Er:fiber pump. For further pulse
energy scaling we also realized the chirped-pulse regime, with 0.8-2 ps pulse durations. The demonstrated applications of such mid-IR source range from extra- and intra-cavity spectroscopy to subharmonic OPO pumping. For
environmentally-protected delivery we suggest and realize duration-preserving soliton delivery in a ZBLAN fiber.
Further bandwidth increase is demonstrated by 2.0-2.8 μm supercontinuum generation in a chalcogenide fiber.
We demonstrate mid-infrared (mid-IR) supercontinuum generation with bandwidth from 2 to 2.8 μm at 20 dB below the peak in nonlinear step-index chalcogenide fiber using femtosecond mid-IR pulses directly from the oscillator. We compare the results with a supercontinuum generated in a silica-based high germanium content fiber. Supercontinuum generation occurs at 90 mW of launched average pump power that is equal to the 0.9 nJ pulse energy. The distinctive feature of the obtained supercontinuum is its stability and coherence due to the deterministic supercontinuum generation by the femtosecond pump pulses
A Thulium fiber laser pumped or InP diode laser stack pumped Cr:ZnSe thin disk cw multimode laser at 2.4 μm with an output power of 5 and 4 W, respectively, and with optical-tooptical efficiencies of 10% will be presented. An experimentally verified and numerically simulated thermal lensing induced and cyclic instability in the laser system will be shown. As a consequence, in order to prevent the lasing conditions in the resonator to be unstable, power scaling of a Cr:ZnSe thin disk laser is possible by enlarging the pump spot and reducing thereby the thermal lensing condition. Therefore, the instability is not initiated. As a conclusion, the investigated instability will show up in any laser active material which has a strong absorption of the pump beam, for instance in transition metal ion laser material systems in connection with any laser concept, like for instance in thin disk, bulk or slab designs.
We demonstrate the novel picosecond mode-locked Y<sub>2</sub>O<sub>3</sub>-codoped Yb/Tm-doped fiber lasers, operating at 1950 nm and producing pulses of up to 1 nJ energy, using a SESAM and an Er-doped pump fiber laser operating at the wavelength 1590 nm or a semiconductor pump laser operating at the wavelength of 1560 nm. We also report on the spectroscopic characterization of these new fibers with various compositions, identifying the optimum one for the maximum Yb/Tm energy transfer, the latter increasing with the increase of the Y concentration. The observed energy transfer between Yb and Tm makes this laser promising also for direct diode-pumping with most advanced and low cost 975 nm diodes, making this laser attractive for compact low cost picosecond Tm-doped fiber laser systems.
The nonlinear refractive index, <i>n<sub>2</sub></i>, of single crystal ZnSe was characterized in the 1200-1950 nm wavelength region using the z-scan technique with picosecond pulses provided by a widely tunable traveling-wave optical parametric amplifier. We have found that the <i>n<sub>2</sub></i> values range from ~15.8×10<sup>-6</sup> to ~9.3×10<sup>-6</sup> cm<sup>2</sup>/GW. The measured spectrum and scaling of the nonlinear refractive index complements previously reported values at shorter wavelengths, and is in good agreement with a theoretical model based on the nonlinear Kramers-Kroenig transformation.
A number of factors that influence spectral position of the femtosecond pulse in a Kerr-lens modelocked Cr:LiSGaF laser have been identified: high-order dispersion, gain saturation, reabsorption from the ground state, and stimulated Raman scattering. Using the one-dimensional numerical model for the simulation of the laser cavity, the relative contributions of different factors have been compared. The Raman effect provides the largest self- frequency shift from the gain peak (up to 60 nm), followed by the gain saturation (approximately 25 nm), while the high-order dispersion contribution is insignificant (approximately 5 nm). Comparison with the experimental data confirm that the stimulated Raman scattering is a main cause of the ultrashort pulse self-frequency shift observed in Cr:LiSGaF and Cr:LiSAF lasers.
We report on the results of lasing and spectroscopic investigations of the anomalously slow recoverable bleaching of Cr,Ca:YAG and Cr,Mg:YAG crystals. We propose two models for this effect: (i) Cr<SUP>4+</SUP> reduction to Cr<SUP>3+</SUP> due to oxygen valence band electron capture by Cr<SUP>4+</SUP>, and (ii) induced disorder in Cr<SUP>4+</SUP> tetrahedral center due to effect of oxygen vacancies.
We present a review of our work on mirror-dispersion- controlled (MDC) Kerr-lens mode-locked (KLM) Cr:LiSGaF and Cr:LiSAF lasers, aimed at studying nonlinear phenomena in the 15-fs regime. Such effects as pulse self-frequency shift a side-band generation due to high-order dispersion (HOD), are looked at in more detail. These phenomena take place in any crystalline solitary mode-locked oscillator, and represent important limitations towards achieving ultimately short pulse durations.